<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            display: flex;
            align-items: stretch;
            /* 侧轴拉伸 */
            /* 弹性盒子在侧轴方向没有尺寸才能拉伸 */

            align-items: center;
            /* 侧轴居中 */

            align-items: flex-start;
            /* 侧轴起点开始排列 */

            align-items: flex-end;
            /* 侧轴终点开始排列 */

            height: 300px;
            border: 1px solid #000;
        }

        /* 控制第二个div侧轴居中对齐 */
        .box div:nth-child(2) {
            align-self: center;
        }

        .box div {

            width: 200px;
            /* height: 100px; */
            background-color: pink;
        }
    </style>
</head>

<body>
    <div class="box">
        <div>1</div>
        <div>2</div>
        <div>3</div>
    </div>

</body>

</html>